So pilots: when on the ground and turning the aircraft, why does the rudder (I think that is what it is) on the tail move? Also what purpose does it play whilst in the air?
Ah, the joys of flight controls. The elevator controls pitch. The ailerons control roll. And the rudder controls yaw. Other controls come into it as well (just to complicate the issue). Spoilers also control roll. The flying tail of most airliners is there for trim, but any movement also results in pitch. And then things get a bit more complex, as controls also tend to have secondary effects. A rudder input will cause yaw, but will also cause roll in the same direction. An aileron input will cause some yaw, but this time in the adverse (i.e. wrong) direction. Spoilers will also cause some yaw, but in the correct direction.
Whilst yaw is how you drive your car around, it is not how you steer an aircraft. Yaw is felt as a quite uncomfortable lateral motion. We want to fly the aircraft so that it is 'in balance'...so that it is pointed straight into the air flow. A turn involves banking, which has the effect of pointing the lift vector (remember that from high school) into the turn. The components of the lift vector will then break down into a part pointing into the turn, and a component in the vertical (which is the bit that keeps you flying). As you enter the bank, slightly more total lift is required (so that the vertical component remains equal to the aircraft weight for a level turn), which means you need a little nose up pitch and a little more power. At this point I haven't mentioned the rudder at all, because only a tiny input is needed in any turn, and in most airliners this is done automatically by a yaw damper system. Even without a yaw damper, you can pretty much treat the rudders as foot rests for 99% of the time.
But...on the ground, we don't have the ability to bank the aircraft. So, when taking off or landing, the steering is accomplished by a mix of nose gear steering and rudder. Generally the rudder will have enough airflow over it to have more steering power than the nose gear at speeds above about 70 knots. We don't change from using one system to another though. The two are interlinked through the rudder pedals, so that on the ground, full rudder deflection results in about 7º of nose gear steering displacement. The steering tiller (which we don't use on the runway) will give about 70º of n/g displacement. The upshot is that you'll often see very large rudder displacements near the start of a take off roll, simply because the rudder is ineffective, and the interconnect gives limited nose gear steering...so a fairly mild 3.5º n/g steering input would also result in half rudder deflection.
Crosswinds...taking off in a crosswind can require large amounts of rudder, as the aircraft will want to weather cough into the wind...whereas we'd rather have it stay on the runway. Landing in a crosswind, the rudder is used during/after the flare to point the aircraft down the runway, and out of the airflow.
Engine failure....will leave you with a large amount of thrust on one side of the aircraft, and a lot less, or none, on the other. That unbalanced thrust will try to yaw the aircraft, and it has to be countered by the rudder. Most airliners will have sufficient rudder authority to counter that thrust on the ground from a speed of around 120 knots...known as Vmcg (minimum control ground). Airborne that speed is slightly lower because a small amount of bank can also be used. With two engines out on the same side, the 380 has sufficient rudder authority to counter full power down to 144 knots. An engine failure at just over V1 (and continue) in the sim, would require pretty well full deflection. After you'd flown away, cleaned up and accelerated, that would reduce to about ¼ deflection...but it will have to be adjusted for every speed and power change....
